1. Field of the Invention
This invention relates to information mark display devices, and, more particularly, to information mark display devices of the type in which a mark representing information to be displayed is formed in a portion of at least one surface of a transparent substrate plate and the display of the mark is presented by controlling the illumination light arrangement.
2. Description of the Prior Art
It is already known in the art to provide a wide variety of information mark display devices as the means for displaying different types of information in the field of view of the finder in a camera or the like, by which the required information display is presented when the necessity arises as the mark appears to ovelap the object image. By the way, in application of such information mark display device to the finder-built-in information display means in a camera or the like, it is thought necessary that when in the normal or display-absent position, the provision of the information mark display presenting portion should not contribute to any bad influence such as so-called "darkening" on the appearance of an intrinsic image to be observed in the field of view of the finder, that is, the object image. From this point of view, the applicant of this application has already proposed such information mark display device in the two forms, one of which makes use of diffraction means comprising a volume hologram in the information mark presenting portion thereof, and the other of which makes use of diffraction means comprising a relief hologram, in Japanese Patent Applications No. Sho 51-106886 and No. Sho 53-92234 respectively. According to these proposals, the mark is recorded on a transparent substrate in various forms of a diffraction grating depending upon the informations to be displayed. When the diffraction grating mark is illuminated, the diffracted light goes to the axis of observation (that is, in the case of the camera, to the direction of the finder optical axis) so that the desired display of the information mark is presented as overlapping an image to be observed (that is, the finder image). The characteristic feature of this diffraction grating type information mark display device is that when the illumination light is not given, the mark bearing portion exerts no bad influence on the image to be observed (the finder image). In practice, this is realized by limiting the efficiency of the diffraction grating to a low value with the advantage that an image bearing light beam which is to provide the finder image is not so much diffracted as to darken the image or to produce the so-called "obscuring" phenomenon. In actual practice, however, the diffraction grating must be of a very fine pattern on the order of 1000 lines/mm as determined by the relation between the angle of incidence of the illumination light beam on the mark and the angle of emergence of the information bearing light reproduced thereby. Therefore, high-precision manufacturing techniques are required.
On the other hand, even with the use of the information mark device as the means of displaying information in the camera finder, it is desired in some cases, depending upon what information is to be displayed, that even when the above-described information mark display presenting portion is deprived of the illumination light from the illumination light source, it is left sufficiently visible, thus functioning to display an alternate information. For example, an auto-focus camera is selected to employ such information mark display device for the purpose of indicating when the photo-taking lens is sharply focused. In this case, it is very significant that the mark pattern of that information mark display presenting portion itself is made to also serve as a mark for the effective area of view-field of focus detection within the object space. In such case, it is required that even when the information mark display presenting portion is deprived of the illumination light from the illumination light source, the mark pattern of that information mark display presenting portion remains visible in the field of view of the finder. This is because even when in the display absent condition, the mark pattern of the above-described information mark display presenting portion is seen as "obscuring" in the image to be observed and the focus detecting area is always displayed to make it possible to perform the focus detecting operation. As will be understood from this fact, upon consideration of a suitable method of using the information mark device, the obscuring phenomenon produced by the mark pattern of the information mark display presenting portion can be advantageously utilized. Therefore, it may be said that the "obscuring" is allowed to take place to some extent when in the display absent condition, provided that it is used for a particular purpose.